The purpose of this project is to conduct statistical research and provide consultation to the Division for clinical trials, laboratory experiments, and epidemiological studies relevant to cancer prevention and control. Research problems under investigation include statistical models for monitoring prevention trials; design of nutritional cohort studies including two-stage designs that allow construction of a followup cohort with a greater variation of nutrient intake; design of studies to calibrate dietary assessment instruments, allowing repeated assessments of two different instruments, one of which is unbiased; use of Bayesian methods for monitoring clinical trials; studies on a large observational database of HIV-infected subjects with evaluation of natural history of disease; and the design of trials for the prevention of recurrence of polyps. Statistical consultation is provided to numerous studies including the NIH Women's Health Initiative Clinical Trial and the Polyp Prevention Trial. The consultation has involved extensive contributions to study design, study analysis and in the case of the Polyp Prevention Trial, continual advice on the day-to-day operations and on data monitoring.